My wife's Toyota Yaris hatch back, battery is draining overnight, I consulted a mechanic, he checked the battery by using volt meter, and told me the battery is fine, and then removed the negative terminal while the engine is running, after removing the terminals also the engine was running so he told me alternator is also fine, and told me that there is some equipment in the car which is draining the battery, he then removed the battery negative terminal and connected a bulb in between them it was lighting, and told me to leave the car for detailed checking, please advise me if there is any way to easily diagnose this issue?

"and then removed the negative terminal while the engine is running." - I don't think your "mechanic" knows what he's doing. If there HAD been an alternator or rectifier fault, doing that could have destroyed the ECU and probably other things as well. The fact that he connected a "bulb" and not a meter also suggests he's not behaving like a professional here! I would take the car away before he does some expensive damage to it.
– alephzeroJan 8 at 13:42

A professional tech would continue with these steps: connect a meter to measure parasitic current draw. Then methodically remove fuses while monitoring the current. When the draw returns to normal, that fuse circuit is the faulty one. If no fuses show the problem, use a wiring diagram to isolate non-fused circuits such as alternator, power seats, starter circuit, etc.
– John CanonJan 9 at 2:34